Diabetes refers to a disease state or process derived from multiple causative factors and is characterized by elevated levels of plasma glucose (hyperglycemia) in the fasting state or after administration of glucose during a glucose tolerance test. Persistent or uncontrolled hyperglycemia is associated with a wide range of pathologies. Diabetes mellitus, is associated with elevated fasting blood glucose levels and increased and premature cardiovascular disease and premature mortality. It is also related directly and indirectly to various metabolic conditions, including alterations of lipid, lipoprotein, apolipoprotein metabolism and other metabolic and hemodynamic diseases. As such, the diabetic patient is at increased risk of macrovascular and microvascular complications. Such complications can lead to diseases and conditions such as coronary heart disease, stroke, peripheral vascular disease, hypertension, nephropathy, neuropathy, and retinopathy. Accordingly, therapeutic control and correction of glucose homeostasis is regarded as important in the clinical management and treatment of diabetes mellitus.
There are two generally recognized forms of diabetes. In type 1 diabetes, or insulin-dependent diabetes mellitus (IDDM), the diabetic patient's pancreas is incapable of producing adequate amounts of insulin, the hormone which regulates glucose uptake and utilization by cells. In type 2 diabetes, or noninsulin dependent diabetes mellitus (NIDDM), patients often produce plasma insulin levels comparable to those of nondiabetic subjects; however, the cells of patients suffering from type 2 diabetes develop a resistance to the effect of insulin, even in normal or elevated plasma levels, on glucose and lipid metabolism, especially in the main insulin-sensitive tissues (muscle, liver and adipose tissue).
Insulin resistance is not associated with a diminished number of cellular insulin receptors but rather with a post-insulin receptor binding defect that is not well understood. This cellular resistance to insulin results in insufficient insulin activation of cellular glucose uptake, oxidation, and storage in muscle, and inadequate insulin repression of lipolysis in adipose tissue, and of glucose production and secretion in the liver. A net effect of decreased sensitivity to insulin is high levels of insulin circulating in the blood without appropriate reduction in plasma glucose (hyperglycemia). Hyperinsulinemia is a risk factor for developing hypertension and may also contribute to vascular disease.
The available treatments for type 2 diabetes, some of which have not changed substantially in many years, are used alone and in combination. Many of these treatments have recognized limitations, however. For example, while physical exercise and reductions in dietary intake of fat, high glycemic carbohydrates, and calories can dramatically improve the diabetic condition, compliance with this treatment is very poor because of well-entrenched sedentary lifestyles and excess food consumption, especially of foods containing high amounts of saturated fat. Increasing the plasma level of insulin by administration of sulfonylureas (e.g. tolbutamide and glipizide) or meglitinide, which stimulate the pancreatic beta-cells to secrete more insulin, and/or by injection of insulin when sulfonylureas or meglitinide become ineffective, can result in insulin concentrations high enough to stimulate insulin-resistance in tissues. However, dangerously low levels of plasma glucose can result from administration of insulin or insulin secretagogues (sulfonylureas or meglitinide), and an increased level of insulin resistance due to the even higher plasma insulin levels can occur. The biguanides are a separate class of agents that can increase insulin sensitivity and bring about some degree of correction of hyperglycemia. These agents, however, can induce lactic acidosis, nausea and diarrhea.
The glitazones (i.e. 5-benzylthiazolidine-2,4-diones) are another class of compounds that have proven useful for the treatment of type 2 diabetes. These agents increase insulin sensitivity in muscle, liver and adipose tissue in several animal models of type 2 diabetes, resulting in partial or complete correction of the elevated plasma levels of glucose without occurrence of hypoglycemia. The glitazones that are currently marketed are agonists of the peroxisome proliferator activated receptor (PPAR), primarily the PPAR-gamma subtype. PPAR-gamma agonism is generally believed to be responsible for the improved insulin sensititization that is observed with the glitazones. Newer PPAR agonists that are being tested for treatment of Type II diabetes are agonists of the alpha, gamma or delta subtype, or a combination thereof, and in many cases are chemically different from the glitazones (i.e., they are not thiazolidinediones). Serious side effects (e.g. liver toxicity) have been noted in some patients treated with glitazone drugs, such as troglitazone.
Compounds that are inhibitors of the dipeptidyl peptidase-IV (DPP-IV) enzyme are also under investigation as drugs that may be useful in the treatment of diabetes, and particularly type 2 diabetes.
Additional methods of treating hyperglycemia and diabetes are currently under investigation. New biochemical approaches include treatment with alpha-glucosidase inhibitors (e.g. acarbose) and protein tyrosine phosphatase-1B (PTP-1B) inhibitors.
Other approaches to treating hyperglycemia, diabetes, and indications attendant thereto have focused on the glucagon hormone receptor. Glucagon and insulin are the two primary hormones regulating plasma glucose levels. Insulin, released in response to a meal, increases the uptake of glucose into insulin-sensitive tissues such as skeletal muscle and fat. Glucagon, which is secreted by alpha cells in pancreatic islets in response to decreased postprandial glucose levels or during fasting, signals the production and release of glucose from the liver. Glucagon binds to specific receptors in liver cells that trigger glycogenolysis and an increase in gluconeogenesis through cAMP-mediated events. These responses generate increases in plasma glucose levels (e.g., hepatic glucose production), which help to regulate glucose homeostasis.
Type 2 diabetic patients typically have fasting hyperglycemia that is associated with elevated rates of hepatic glucose production. This is due to increased gluconeogenesis coupled with hepatic insulin resistance. Such patients typically have a relative deficiency in their fasting and postprandial insulin-to-glucagon ratio that contributes to their hyperglycemic state. Several studies have demonstrated that hepatic glucose production correlates with fasting plasma glucose levels, suggesting that chronic hepatic glucagon receptor antagonism should improve this condition. In addition, defects in rapid postprandial insulin secretion, as well as ineffective suppression of glucagon secretion, lead to increased glucagon levels that elevate hepatic glucose production and contribute to hyperglycemia. Suppression of elevated postprandial glucagon levels in type 2 diabetics with somatostatin has been shown to lower blood glucose concentrations. This indicates that acute postprandial glucagon receptor antagonism would also be beneficial. Based on these and other data, glucagon receptor antagonism holds promise as a potential treatment of type 2 diabetes by reducing hyperglycemia. There is thus a need in the art for small-molecule glucagon receptor antagonists with good safety profiles and efficacy that are useful for the treatment of hyperglycemia, diabetes, and related metabolic diseases and indications. The present invention addresses that need.